Mercenary Guild - This is in the City Core, where you go to hand in those severed heads. Talk to Karin the receptionist about it. Anyone under level 4 can join, but if you're higher than that level, having cleric, paladin, sorcerer, or wizard levels will get you rejected. You'll then need a DM help to join. At least, as far as I know, that's how it seems to work.